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Daniel, If I were you I would put some plates om the inside of the boom extending at least 8" eather side of the crack, I would then take torch cut a groove length wise or parallel to the boom and plug weld the plate.

Note, Theres more pressure being applied to the top and bottom of the boom so thats where you want your fish plates.

2 plates welded to the top corners and 2 plates welded to the bottom corners.

I would then install the backers as you've done between the flat bars that run the length of the boom.

I know its more common to see the heavy fish plates on the sides of the boom, butt you actualy get more strength when putting fish plates on the top and bottom flange of the boom.
Other than where you splice the new section of boom in, Try to minimize welding across the boom.

Good luck.

Are you saying that fish plates on top and bottom only are stronger than on the sides?

fish paltes top and bottom...

wow! that looks like a fun project.

i'm absolutely no expert but it seems to me too like the fish plates on top and bottom would make a stronger repair...looks like they would keep the seams from "stretching" apart when the cylinder applies the force.

i look forward to seeing more pictures!

sometimes i wish i could quit my "real job" and do that kind of stuff, but i kind of like the climate controlled environment of the hospital!

After talking about it for a bit with the owner, every joint made on the boom has a backing strip, every joint are welded right thru with a full penetration all the way in the backing. It will make everything sound solid.
So all this done will make the almost as strong as it was when it was new.
Then after I going to put a fishplate on the top and it will go cover both joint, I will put a fishplate on both side of the boom, these ones will extend pass both joints about 12 inches with a bunch of plug welds on them.
These will make the boom stronger then new.
I'll post some picks tomorrow after work, today it was going around with the root passes and adding backing on the edges where the top and bottom plates extend pass the side plate,
Still have more passes to go before fishplating